The limit test for iron is a qualitative test that checks whether the iron ion (Fe³⁺) concentration in a sample is below a specified limit.
It is based on a colorimetric reaction between iron ions (Fe³⁺) and potassium thiocyanate (KSCN), forming a red-colored complex.
Chemical Reaction:
Fe³⁺ (aq) + 6 SCN⁻ (aq) → [Fe(SCN)₆]³⁻ (aq)
Procedure of Limit test for Iron:
1) Prepare Reference Standard Solution:
Make a reference solution with a known concentration of iron ions, typically from ferric ammonium sulfate.
This solution represents the acceptable iron limit.
2) Prepare Test Solution:
Prepare a test solution from the sample, ensuring that any iron ions present are in a soluble form.
3) Add Potassium Thiocyanate (KSCN):
To both the test and reference solutions, add equal volumes of potassium thiocyanate solution to provide the thiocyanate ions for the reaction.
4) Allow Formation of Red Complex:
Let both solutions stand for a few minutes to allow the red-colored iron-thiocyanate complex to form.
5) Observe Color Formation:
Observe the intensity of the red color in both the test and reference solutions.
The presence of a red color in the test solution indicates iron ions.
6) Compare Color Intensity:
Compare the intensity of the red color in the test solution with the reference standard.
If the test solution shows less intense color, the iron ion concentration is below the acceptable limit.